A transmitter of a key base station in a mobile communication system requires a power amplifier having high efficiency and high linearity. At the present, in order to amplify many signal channels at once, a power amplifier having high linearity is especially required. In order to increase the linearity of the power amplifier, in addition to employing a distortion compensator such as a feed forward system, it is necessary to reduce the distortion of an output of the power amplifier.
Meanwhile, an optical forward base station is placed as a base station in an area where communication traffic is heavy, for example an urban area in order to cover a communication area secondarily. This optical forward base station converts an optical signal transmitted from a key base station through an optical fiber to a high-frequency signal and transmits it. The optical forward base station is required to be small in size.
A wireless communication base station system including the optical forward base station will be described below with reference to FIG. 1 which is a block diagram of a wireless communication base station system including a conventional optical forward base station. As shown in FIG. 1, a wireless key base station 101 and an optical forward base station 102 are connected to each other through optical fibers 103, 104. The wireless key base station 101 consists of an input terminal 110 for a baseband signal, an output terminal 111 for a baseband signal, a modulator 112, a demodulator 113, an electrical to optical converter 114, and an optical to electrical converter 115. The optical forward base station 102 consists of an optical to electrical converter 120, an electrical to optical converter 121, a power amplifier 122, an antenna duplexer 123, an antenna 124, and a low-noise amplifier 125.
When a signal is transmitted from the wireless key base station 101 to the optical forward base station 102, if a baseband signal is input to the input terminal 110 for a baseband signal in the wireless key base station 101, the baseband signal is converted to a high-frequency signal by the modulator 112. The high-frequency signal is converted to an optical signal by the electrical to optical converter 114 and then transmitted to the optical forward base station 102 through the optical fiber 103. The transmitted optical signal is converted to a high-frequency signal by the optical to electrical converter 120 in the optical forward base station 102. The high-frequency signal is amplified by the power amplifier 122 to a desired output level. Thereafter, the amplified signal is radiated from the antenna 124 through the antenna duplexer 123. In this case, the power amplifier 122 is generally operated in a high-linearity area to make an adjacent-channel leakage power level equal to or lower than a predetermined value.